The wheel and tyre Bible - everything you need to know about how to read your tyres (tires) and wheels with more info on rim sizes, tread depth and wear, aquaplaning, wheel balancing, aftermarket wheels, alloys, TPMS tire pressure monitoring systems and much more.

The Wheel & Tyre Bible

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04/21/2014 07:00 AM
Faster really is safer.
From Auto Express recently:
MOTORING groups have backed a Danish report which claims that increasing speed limits is safer – but there are question marks over whether it could be easily implemented in the UK.
The study, carried out over two years by the Danish Road Directorate, looked at how driver behaviour and accident rates changed when speed limits were raised on single-carriageway rural roads and motorways.
One of the key findings was that after raising limits on two-way rural roads from 50mph to 56mph, accidents fell, due to a drop in the speed differential between the fastest and the slowest drivers, resulting in less overtaking. While the slowest drivers increased speeds, the fastest 15 per cent were found to be driving 1mph slower on average.
On sections of motorways where the limit was raised from 68mph to 80mph nine years ago, fatalities also fell.
A spokesman for the Alliance of British Drivers told Auto Express: "The research would seem to suggest that we are going the wrong way in the UK. This has proven that deaths and accidents have fallen despite limits increasing."
A Transport Research Laboratory (TRL) spokesman said the research raised interesting questions. "A key element isn't just the risk of the crash that is proportional to travelling speed for a given road, but the risk of injury should a collision occur." The Association of Chief Police Officers would not comment.

This isn't the first time a study like this has been published. There have been plenty of trials of increased speed limits where accident rates have fallen. The reason is simple : people are already driving quickly. Often quicker than the current limits dictate. Occasionally they'll come across people who are doing the limit - and it's that differential that causes the problem. The quicker person wants to get past - normally meaning an overtake. The more impatient they get, the more likely they'll do something silly, and the greater the liklihood of an accident. So what happens when the speed limit is raised? Those already travelling quicker will tend to stay at the same speeds they used to, whilst those who like to travel exactly at the limit will go quicker - to the new higher limit - thus reducing the differential in speed between them and everyone else.
I'm all for higher speed limits - the trend in the UK to keep slowing people down is clearly not working (see my post a few months back about how slower limits cause more accidents). And remember - the educated motorist understands that speed doesn't kill. It's the inappropriate use of speed that can amplify the end result of an accident.

Are you confused by your car tyres, or if you're American, your tires? Don't know your rolling radius from your radial? Then take a good long look through this page where I hope to be able to shift some of the mystery from it all for you. At the very least, you'll be able to sound like you know what you're talking about the next time you go to get some new tyres.

How to read your tyre markings

This is probably the number one question I get asked - "how do I read my tyre markings?". It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actually, most of that information in a tyre marking is surplus to what you need to know. So here's the important stuff:

How to read your tire markings

[tyre markings]
Key Tyre Marking Description
A Manufacturers or brand name, and commercial name or identity.
B Tyre size, construction and speed rating designations. Tubeless designates a tyre which requires no inner tube. See tyre sizes and speed ratings below. DIN-type tyre marking also has the load index encoded in it. These go from a load index of 50 (190kg) up to an index of 169 (5800kg).
C Denotes type of car tyre construction.
D M&S denotes a car tyre designed for mud and snow. Reinforced marking only where applicable.
E Pressure marking requirement.
F ECE (not EEC) type approval mark and number.
G North American Dept of Transport compliance symbols and identification numbers.
H Country of manufacture.

As well as all that, you might also find the following embossed in the rubber tyre marking:

Encoded in the US DOT information (G in the tyre marking above) is a two-letter code that identifies in detail where the tyre was manufactured. In other words, what factory and in some cases, what city it was manufactured in. It's the first two letters after the 'DOT' - in this case "FA" denoting Yokohama.
This two-letter identifier is worth knowing in case you see a tyre recall on the evening news where they tell you a certain factory is recalling tyres. Armed with the two-letter identifier list, you can figure out if you are affected. It's a nauseatingly long list, and I've not put it on this page. But if you click here it will popup a separate window with just those codes in it.

Additional markings

In addition to all of the above, here is a comprehensive list of other markings you can find on your sidewall.
This section is hidden by default because it takes up a lot of space on the page. expand/contract the additional markings section.

In additions (yes, there's more), these are what you'll find on run-flat tyres:

OE Manufacturer Letters

In the same way that Porsche specifies N-rated tyres (see later), there's even more markings on the sidewall of a tyre that can denote tyres fitted as original equipment (OE) to various makes of vehicle. In some cases, the 'preferred' OE tyres are slightly different than the same-named tyres without the OE specification. For example on some Honda SUVs, the tyres that are stamped with 'DZ' have a lower rolling resistance tread pattern than those that aren't. In true "Keeping the customer confused" fashion, you'll notice that (F) is not the same as (f). Go (f)igure. The following table shows all the letters assigned to OE tyres for various manufacturers.
This section is hidden by default because it takes up a lot of space on the page. expand/contract the OE manufacturer letters section.

CodeManufacturerCodeManufacturer
(-) or -Peugeot / VolvoLOTUSLotus
(*) or *BMW / FiatMMaserati
(1) or 1Bentley / MaseratiM0 / MO Mercedes
1ZToyotaM3BMW M3
5ZFordMBMercedes
(a) or aAudi / Seat / Volvo / VWMZMitsubishi / Toyota
(A) or AAlfa RomeoN0Porsche
AUAudiN1Porsche
AZAlfa Romeo / Fiat / Subaru / ToyotaN2Porsche
BBMWN3Porsche
BMBMWN4Porsche
BZAudi / Honda / Mazda / Mercedes / Opel / Skoda / VWN5Porsche
C1Chrysler / Daimler / Viper(P) of PPeugeot / Porsche
CZSubaruPZFord / Mazda / Mercedes / VW
DCChrysler / DaimlerRRenault
DZHonda / Mazda / ToyotaRERenault
EZAudi / Mazda 6RUBMW / Ferrari / Lexus / Mercedes
(f) or fFiatSMARTSmart
(F) of FFerrari / FordSZMazda MPV
FEFerrariTZMazda 6
FOFordUVVW
G1MercedesUZBMW 3 / Skoda
GZSubaruVolvoVolvo
(J) or JJaguarVWVW
JZSeat / SubaruVW Golf 3VW Golf 3
LLexusVW Golf 4VW Golf 4
LZLanciaXZNissan
K1FerrariYZHyundai / Kia / Mazda / Mercedes
KZNissan / ToyotaZMazda / Toyota

DOT Codes and the 6-year shelf life

tip

As part of the DOT code (G in the tyre marking above), there is a tyre manufacture date stamped on the sidewall. Oddly this code is sometimes only one one sidewall so you might need to get under your car and look at the inward-facing side of the tyre. Take a look at yours - there will be a three- or four-digit code. This code denotes when the tyre was manufactured, and as a rule-of-thumb, you should never use tyres more than 6 years old. The rubber in tyres degrades over time, irrespective of whether the tyre is being used or not. When you get a tyre change, if you can, see if the tyre place will allow you to inspect the new tyres first. It's not uncommon for these shops to have stuff in stock which is more than 6 years old. The tyre might look brand new, but it will delaminate or have some other failure within weeks of being put on a vehicle.
Reading the code. The code is pretty simple. The three-digit code was used for tyres manufactured before 2000. So for example 1 7 6 means it was manufactured in the 17th week of 6th year of the decade. In this case it means 1986. For tyres manufactured in the 90's, the same code holds true but there is a little triangle after the DOT code. So for this example, a tyre manufactured in the 17th week of 1996 would have the code 176triangle
After 2000, the code was switched to a 4-digit code. Same rules apply, so for example 3 0 0 3 means the tyre was manufactured in the 30th week of 2003.

DOT tire code

Check your spare

I had a reader email me about the age code and he pointed out that it's wise to check your spare tyre too. In his case, he had an older vehicle but his running tyres were all nice and fresh. It was his spare that was the problem - it had a date code on it of 081triangle meaning it was manufactured in the 8th week of 1991. At the time of writing, that was a 16 year old tyre. So you've been warned - if you're driving an older car, check the date code of your spare. If you get a flat and your spare is gently corroding in the boot (or trunk), it won't do you much good at all.

DOT Age Code Calculator

The calculation built in to this page is up-to-date based on today's date. If the DOT age code on your tyres is older than this code, change your tyres.

DOT AGE CODE:

Interesting note : in June 2005, Ford and GM admitted that tyres older than 6 years posed a hazard and from their 2006 model year onwards, started printing warnings to this effect in their drivers handbooks for all their vehicles.

Take the age code seriously : a tale of caution

A reader contacted me in 2010 with a tale of caution regarding the manufacturing age code on old tyres.

A tyre manufactured in 1986, failing at 70mph in 2010

In August 2010, I bought a classic 1976 Mercedes with only 30,000 miles on it. The seller (who was only the second owner) warned me that he thought that the Michelin XVS tyres were pretty old (the spare was unused). I was aware of the dangers of old tyres from reading your tyre bible, but it was a Sunday and the tyres are an unusual size (205/70R14) and were not readily available. I thought that I'd risk the trip back home (250 miles), but that I'd need to get new tyres ASAP. Unfortunately, one of the tyres didn't last that long, and failed at 70 MPH (see photo, note my skidmarks). It turned out that the tyres were date-stamped from week 30, 1986(!), so the advice about old tyres is indeed true!
For the record, I ordered new tyres and had the minor body damage repaired, and all is well with the car now. Stephen W, Dublin, Ireland.

The E-Mark

e-mark

Item F in the tyre marking diagram above is the E-mark. All tyres sold in Europe after July 1997 must carry an E-mark. The mark itself is either an upper or lower case "E" followed by a number in a circle or rectangle, followed by a further number.
An "E" (upper case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of ECE regulation 30.
An "e" (lower case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of Directive 92/33/EEC.
The number in the circle or rectangle denotes the country code of the government that granted the type approval. 11 is the UK. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tyre size and type.

Tyre size notations

Okay, so you look at your car and discover that it is shod with a nice, but worn set of 185-65HR13's (from the tyre marking). Any tyre mechanic will tell you that he can replace them, and he will. You'll cough up and drive away safe in the knowledge that he's just put some more rubber on each corner of the car that has the same shamanic symbols on it as those he took off. So what does it all mean?

tire size markings

tyre size notation
This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on (or top-down). This is known as the section width. This is the ratio of the height of the tyre sidewall, (section height), expressed as a percentage of the width. It is known as the aspect ratio. In this case, 65% of 185mm is 120.25mm - the section height. This is the speed rating of the tyre. This tells you that the tyre is a radial construction. Check out tyre construction if you want to know what that means. This is the diameter in inches of the rim of the wheel that the tyre has been designed to fit on. Don't ask me why tyre sizes mix imperial and metric measurements. They just do. Okay?

More recently, there has been a move (especially in Europe) to adjust tyre designations to conform to DIN. This is the German Institute for standardisation - Deutsches Institut fuer Normung, often truncated to Deutsche Industrie Normal. DIN sizing means a slight change in the way the information is presented to the following:

tyre size notation
Section width Aspect ratio Radial Rim diameter Load index Speed rating.

Ultra high speed tyre size notations

There is a subtle difference in the notation used on ultra high speed tyres, in particular motorcycle tyres. For the most part, the notation is the same as the DIN style described above. The difference is in the way the speed rating is displayed. For these tyres, if the speed rating is above 149mph, then a 'Z' must appear in the dimension part of the notation, as well as the actual speed rating shown elsewhere. The 'Z' is a quick way to see that the tyre is rated for over 149mph.

tyre size notation
Section width Aspect ratio 149+ mph rated Radial Rim diameter Load index Speed rating.

Classic / vintage / imperial crossply tyre sizes

What ho. Fabulous morning for a ride in the Bentley. Problem is your 1955 Bentley is running on 7.6x15 tyres. What, you ask, is 7.6x15? Well it's for older vehicles with imperial measurements and crossply tyres. Both measurements are in inches - in this case a 7.6inch tyre designed to fit a 15inch wheel. There is one piece of information missing though - aspect ratio. Aspect ratios only began to be reduced at the end of the 1960s to improve cornering. Previously no aspect ratio was given on radial or crossply tyres. For crossply tyres, the initial number is both the tread width and the sidewall height. So in my example, 7.6x15 denotes a tyre 7.6 inches across with a sidewall height which is also 7.6 inches. After conversion to the newer notation, this is the equivalent to a 195/100 15. If you're plugging numbers into the tyre size calculator lower down this page, I've included an aspect ratio value of 100 for imperial calculations.
Note: I put 195/100 15 instead of 195/100R15 because technically the "R" means radial. If you're trying to get replacement crossply tyres, the "R" won't be in the specification. However if you're trying to replace your old crossply tyres with metric radial bias tyres, then the size does have the "R" in it. Here is a javascript calculator to turn your imperial tyre size into a radial metric tyre size:

Your imperial tyre size: x
Equivalent standard tyre size is :/100 R

For quick reference, you could also try my vintage tyre size conversion table which lists a lot of common sizes along with their modern counterparts.

classic tire sizes

Classic / vintage radial tyre sizes

Remember above that I said aspect ratios only started to come into play in the 1960s? Unlike the 100% aspect ratio for crossply tyres, for radial tyres, it's slightly different - here an aspect ratio of 80% is be assumed. So for example, if you come across on older tyre with 185R16 stamped on it, this describes a tyre with a tread width of 185mm and a sidewall height which is assumed to be 80% of that; 148mm.
The question of the aspect ratio for radial sizes has been the subject of a lot of email to me. I've had varying figures from 80% up to 85% and everyone claims they're right. Well one reader took it to heart and did some in-depth research. It seem there is actually no fixed standard for aspect ratio when it is not expressly stated in the tyre size. Different manufacturers use slightly different figures.
The english MOT (road-worthiness test) manual states: Unless marked otherwise, "standard" car tyres have a nominal aspect ratio of 82%. Some tyres have an aspect ratio of 80%. These have "/80" included in the size part of the tyre marking e.g. 165/80 R13. Note: Tyres with aspect ratios of 80% and 82% are almost identical in size and can be safely mixed in any configuration on a vehicle.
See http://www.motuk.co.uk/manual_410.htm for the online version.
If you're plugging vintage radial numbers into the tyre size calculator, I've included aspect ratios of 80 and 82 for these calculations.

Alpha numeric load-based tyre sizes for vintage cars

Picture credit: Jensen Intercepter Club

jensen interceptor

On some 60's and 70's era vintage vehicles, (for example the Jensen Interceptor), the tyre sizes were denoted as ER70VR15. The '70' refers to the section height as you might expect, and the '15' is the wheel dimension, but on first inspection there appears to be no section width. Actually there is, but it's in yet another odd format. In this case, the first letter is the thing to look at. The letter itself has no direct equivalent to modern dimensional sizes but instead relates to load index; the higher the letter the more load it can carry. With vintage tyres, higher loads translated into bigger tyres, so the close approximations between old load and new size these days are:
C = 185    D = 195    E = 205    F = 215    G = 225    H = 235 etc.
In this example then, ER70VR15 means 205/70 R15 with a 'V' speed rating. Whilst many of the latter Interceptors were technically capable of 140mph, the aerodynamic behaviour would have you quickly backing off to about 120mph so frankly that 'V' rating is a little optimistic. If you're looking to replace tyres for this type of vehicle, an 'H' speed rated tyre is the better choice, and it's cheaper.
For those of you reading this in the colonies, an example vehicle from this era is the Chevy Nova which had E78-14 tyres. (In this case, there was no letter 'R' meaning these were cross-ply tyres, not radials). The equivalent size in modern notation would be 205/78 R14. The following converter will give you a rough idea of the equivalent metric tyre size for a given alpha numeric tyre size:

Your alphanumeric tyre size: R /R

For quick reference, you could also try my vintage tyre size conversion table which lists a lot of common sizes along with their modern counterparts.

Metric Tyre sizes and the BMW blurb

BMW 525

Fab! You've bought a BMW 525TD. Tyres look a bit shoddy so you go to replace them. What the....? TD230/55ZR390? What the hell does that mean? Well my friend, you've bought a car with metric tyres. Not that there's any real difference, but certain manufacturers experiment with different things. For a while, (mid 1990s) the 525TD came with arguably experimental 390x180 alloy wheels. These buggers required huge and non-conformal tyres. I'll break down that classification into chunks you can understand with your new-found knowledge:
TD - ignore that. 230 = cross section 230mm. 55 = 55% sidewall height. Z=very high speed rating. R390=390mm diameter wheels. These are the equivalent of about a 15.5" wheel. There's a nice standard size for you. And you, my friend, have bought in to the long-raging debate about those tyres. They are an odd size, 180x390. Very few manufacturers make them now and if you've been shopping around for them, you'll have had the odd heart-stopper at the high price. The advice from the BMWcar magazine forum is to change the wheels to standard sized 16" so there's more choice of tyres. 215-55R16 for example. The technical reason for the 390s apparently is that they should run flat in the event of a puncture but that started a whole debate on their forum and serious doubts were expressed. You've been warned...

If you're European, you'll know that there's one country bound to throw a spanner in the works of just about anything. To assist BMW in the confusion of buyers everywhere, the French, or more specifically Michelin have decided to go one step further out of line with their Pax tyre system. See the section later on to do with run-flat tyres to find out how they've decided to mark their wheels and tyres.

Metric tire sizes

Land Rovers and other off-road tyre sizes

book

On older Land Rovers (on the LWB/110 vehicles and many "off-roaders"), you'll often find tyres with a size like 750x16. This is another weird notation which defies logic. In this case, the 750 refers to a decimalised notation of an inch measurement. 750 = 7.50 inches, referring to the "normal inflated width" of the tyre - i.e. the external maximum width of the inflated, unladen tyre. (This is helpfully also not necessarily the width of the tread itself). The 16 still means 16 inch rims. Weird eh? The next question if you came to this page looking for info on Land Rover tyres will be "What size tyre is that the equivalent of in modern notation?". Simple. It has no aspect ratio and the original tyres would likely be cross-ply, so from what you've learned a couple of paragraphs above, assume 100% aspect ratio. Convert 7.5inches to be 190mm. That gives you a 190/100 R16 tyre. (You could use the calculator in the section on Classic / vintage / imperial crossply tyre sizes above to get the same result.)
Generally speaking, the Land Rover folks reckon a 265/65R16 is a good replacement for the "750", although the tread is slightly wider and might give some fouling problems on full lock. It's also 5% smaller in rolling radius so your speed will over-read by about 4mph at 70mph. If you can't fit those, then the other size that is recommended by Landrover anoraks is 235/85R16.
On Discoveries, Range Rovers, or the SWB Defenders/Series land rovers you'll find "205" tyres, denoting 205mm x 16 inches. The 205 type tyres can generally be replaced with 235/70R16 or 225/75R16. The 235 is a wider tyre and the general consensus in Land Rover circles is that it holds the road better when being pushed.
If you're really into this stuff, you ought to read Tom Sheppard's Off Roader Driving (ISBN 0953232425). It's a Land Rover publication first published in 1993 as "The Land Rover Experience". It's been steadily revised and you can now get the current edition from Amazon. I've even helpfully provided you with this link so you can go straight to it....

LT (Light Truck) imperial tyre sizes

Confused yet? Okay how about this: 30x9.5 R15 LT or LT30x9.5/15. Yet another mix-and-match notation, this time for (amongst other things) light truck classification tyres. All the information you need to figure out a standard size is in there, but in the usual weird order. In this case the 30 refers to a 30 inch overall diamter. The 9.5 refers to a 9.5 inch wide tread. The R15 refers to a 15 inch diameter wheel. In order to figure out the closest standard notation, you know the tread width which (in this example) is 9.5 inches or 240mm. The sidewall height is the overall height minus the wheel diameter all divided by 2. So 30 inches minus 15 inches, which gives you 15 inches. Half that to get 7.5 inches and that's the sidewall height - 190mm. Remember the section value is a percentage of the tread width - in this case 190mm/240mm gives us a section of 80% (near enough). So the standard size for 30x9.5R15 works out to be 240/80R15. In truth you can barely find a tyre that size so most off-roaders with that sort of tyre size go for 245/70R15 which is more common. For your convenience, another calculator then.

Your LT tyre size: xR
Equivalent standard tyre size is :/ R

Light truck tires

Porsche N-rated tyres

line of porsche

Porsche designs and manufacturers some of the highest performance cars in the world (with the exception of the butt-ugly Cayenne). All this design and performance is worth nothing if you put cheap Korean tyres on your Porsche, and because of that prospect, Porsche introduced the N rating or N specification system. In order for a manufacturer to be an OE (original equipment) supplier of tyres for Porsches, they must work with the Porsche engineers at the development and testing stage. They concentrate on supreme dry-weather handling but they also spend a considerable amount of time working on wet-weather handling. Porsches are typically very tail-heavy because of the position of the engine relative to the rear wheels, and with traction control off, it's extremely easy to spin one in the wet. Because of this, Porsche specify a set of wet-grip properties which is way above and beyond the requirements of any other car manufacturer.
OE tyres for Porsches must successfully pass lab tests to prove that they would be capable of adequately supporting a Porsche at top speed on a German Autobahn. Once the lab tests are done, they must go on to track and race tests where prototypes are evaluated by Porsche engineers for their high-speed durability, uniformity and serviceability. If they pass all the tests, Porsche give the manufacturer the go-ahead to put the car tyres into production and then they can proudly claim they are an N-rated Porsche OEM (Original Equipment Modifier).
The N-ratings go from 0 (zero) to 4, marked as N-0, N-1 etc. This N-rating, stamped into a tyre sidewall, clearly identifies these tyres as having gone through all the nauseating R&D and testing required by Porsche as described above. The number designates the revision of the design. So for a totally new design, the first approved version of it will be N-0. When the design is improved in some way, it will be re-rated as an N-1. If the design changes completely so as to become a totally new tyre, it will be re-rated at N-0.
If you've got a Porsche, then you ought to be aware that as well as using N-rated tyres, you ought to use matching tyres all around because many Porsches have different sizes tyres front and rear. So for example if you have a Porsche with N-3 rated tyres and the rear ones need replacing but the model has been discontinued, you should not get N-0's and put them on the back leaving the old N-3's on the front. You should replace all of them with the newer-designed re-rated N-0 tyres. But then you own a Porsche so you can certainly afford four new tyres....
One final point. You may go into a tyre warehouse and find two tyres with all identical markings, sizes and speed ratings, but one set has an N-rating. Despite everything else being the same, the non-N-rated tyres have not been certified for use on a Porsche. You can buy them, and you can put them on your car, but if you stuff it into the armco at 150mph, Porsche will just look at you and with a very teutonic expression ask why you didn't use N-rated tyres.

Porsche n-rated tyre

Porsche tires

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Lies, Damn Lies and Speed ratings

All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.

Speed Symbol Max Speed Capability Speed Symbol Max Speed Capability
Km/h MPH Km/h MPH
L12075S180113
M13081T190118
N14087U200125
P15095H210130
Q160100V240150
R170105W270168
Y300186
ZR240+150+

'H' rated tyres are the most commonplace and widely used tyres, having replaced 'S' and 'T' ratings. Percentage-wise, the current split is something like this: S/T=37%, H=63%, V=8%. Certain performance cars come with 'V' or 'ZR' rated tyres as standard. This is good because it matches the performance capability of the car, but bad because you need to re-mortgage your house to buy a new set of tyres.
A note on the ZR rating: most speed ratings designate the speed that should not be exceeded. For example W-rated tyres are rated up to 270km/h. ZR-rated tyres, by comparison, are theoretically rated for anything over 240km/h. This is why the Bugatti Veyron was shod with ZR-rated tyres for the world record speed run of 431km/h when Top Gear drove it at the VW Ehra-Lessien test track. Although to be honest, at that speed the tyres only lasted for 8 minutes.

UTQG Ratings

The UTQG - Uniform Tyre Quality Grade - test is required of all dry-weather tyres ("snow" tyres are exempt) before they may be sold in the United States. This is a rather simple-minded test that produces three index numbers : Tread life, Traction and Temperature.

There are some exceptions: Yokohama A008's are temperature rated "C" yet are sold as "H" speed rated tyres. These UTQC tests should be used only as a rough guide for stopping. If you drive in the snow, seriously consider a pair of (if not four) "Snow Tyres". Like life, this tyre test is entirely subjective.

Load indices

The load index on a tyre is a numerical code associated with the maximum load the tyre can carry. These are generally valid for speed under 210km/h (130mph). Once you get above these speeds, the load-carrying capacity of the tyre decreases and you're in highly technical territory the likes of which I'm not going into on this page. (Mostly because I don't understand it).
The table below gives you most of the Load Index (LI) values you're likely to come across. For the sake of simplicity, if you know your car weighs 2 tons - 2000kg - then assume an even weight on each wheel. 4 wheels at 2000kg = 500kg per wheel. This is a load index of 84. The engineer in you should add 10% or more for safety's sake. For this example, I'd probably add 20% for a weight capacity of 600kg - a load index of 90. Generally speaking, the average car tyre is going to have a much higher load index than you'd ever need. It's better to have something that will fail at speeds and stress levels you physically can't achieve, than have something that will fail if you nudge over 60mph with a six pack in the trunk.

LI   kg
 50  190 
51  195
 52  200 
53  206
 54  212 
55  218
 56  224 
57  230
 58  236 
59  243
 60  250 
61  257
 62  265 
63  272
 64  280 
65  290
 66  300 
67  307
 68  315 
69  325
 
LI   kg
 70  335 
71  345
 72  355 
73  365
 74  375 
75  387
 76  400 
77  412
 78  425 
79  437
 80  450 
81  462
 82  475 
83  487
 84  500 
85  515
 86  530 
87  545
 88  560 
89  580
 
LI   kg
 90  600 
91  615
 92  630 
93  650
 94  670 
95  690
 96  710 
97  730
 98  750 
99  775
 100  800 
101  825
 102  850 
103  875
 104  900 
105  925
 106  950 
107  975
 108  1000 
109  1030
 
LI   kg
 110  1060 
111  1090
 112  1120 
113  1150
 114  1180 
115  1215
 116  1250 
117  1285
 118  1320 
119  1360
 120  1400 
121  1450
 122  1500 
123  1550
 124  1600 
125  1650
 126  1700 
127  1750
 128  1800 
129  1850
 
LI   kg
 130  1900 
131  1950
 132  2000 
133  2060
 134  2120 
135  2180
 136  2240 
137  2300
 138  2360 
139  2430
 140  2500 
141  2575
 142  2650 
143  2725
 144  2800 
145  2900
 146  3000 
147  3075
 148  3150 
149  3250
 
LI   kg
 150  3350 
151  3450
 152  3550 
153  3650
 154  3750 
155  3875
 156  4000 
157  4125
 158  4250 
159  4375
 160  4500 
161  4625
 162  4750 
163  4875
 164  5000 
165  5150
 166  5300 
167  5450
 168  5600 
169  5800